You may remember I had to remove small vertical sections from the leading edge of each quarter when I was gapping the body panels a few months ago. With a skim of filler applied to the panel and inside the door shut, wrapping 80-grit around a paint mixing stick ensured the gaps would be even. Don’t be afraid to use whatever will work as a block. Old radiator hoses make excellent blocks for curved areas, for instance. Just make sure they’re clean and free of oil, grease, or silicone.